

package com.hazelcast.topic;

import com.hazelcast.spi.annotation.NamespacesSupported;

import java.util.EventListener;

/**
 * Message listener for {@link ITopic}.
 * <p>
 * A MessageListener will never be called concurrently (provided that it's not registered twice). So there
 * is no need to synchronize access to the state it reads or writes. Also, there is no need to synchronize
 * when publishing to the ITopic, the ITopic is responsible for memory consistency effects. In other words,
 * there is no need to make fields of the MessageListener volatile or access them using synchronized blocks.
 *
 * @param <E> message type
 */
@FunctionalInterface
@NamespacesSupported
public interface MessageListener<E> extends EventListener {

    /**
     * Invoked when a message is received for the topic. Note that the topic guarantees message ordering.
     * Therefore, there is only one thread invoking onMessage. The user should not keep the thread busy, but preferably
     * should dispatch it via an Executor. This will increase the performance of the topic.
     *
     * @param message the message that is received for the topic
     */
    void onMessage(Message<E> message);
}
